(1) Field of the Invention
The present invention relates to a crystalline semiconductor film manufacturing method and a crystalline semiconductor film manufacturing apparatus.
(2) Description of the Related Art
A thin-film transistor called TFT is used for an active matrix driven display apparatus such as a liquid crystal display apparatus or an organic electroluminescence (EL) display apparatus.
In the thin-film transistor, a semiconductor layer which includes silicon or the like and is to be a channel layer is generally formed of an amorphous semiconductor film or a crystalline semiconductor film; however, it is preferable that a semiconductor layer that is to be the channel layer be formed of the crystalline semiconductor film whose mobility is higher than that of the amorphous semiconductor film. Generally, the crystalline semiconductor film is formed by forming an amorphous semiconductor film first and then crystallizing the amorphous semiconductor film.
Methods of forming the crystalline semiconductor film by crystallizing the amorphous semiconductor film includes a laser annealing method such as an excimer laser annealing (ELA) method.
A conventional laser annealing method is disclosed in, for example, Patent Reference 1 (Japanese Unexamined Patent Application Publication No. 4-171717) and Patent Reference 2 (Japanese Unexamined Patent Application Publication No. 11-125839).
The laser beam irradiation method disclosed in Patent Reference 1 involves rotating, relative to each other, the semiconductor substrate and the excimer layer beam. With this, even if the energy density distribution of the laser beam is uneven, the unevenness is offset by the rotation described above, thus equalizing the energy distribution within the surface irradiated with the laser beam.
In addition, the laser annealing method disclosed in Patent Reference 2 is a method that involves swinging a position of the laser beam in a beam scanning direction with each shot. This allows averaging an intensity distribution of the laser beam projected onto the substrate without bias.
It can be considered that crystallizing the amorphous semiconductor film using the laser annealing methods disclosed in Patent References 1 and 2 results in a uniform energy distribution of the laser beam projected onto the amorphous semiconductor film, thus allowing obtaining the crystalline semiconductor film that is uniformly crystallized.
However, in the display apparatus described above which is expected of high display performance such as high image quality, there is a problem of failing to obtain the crystalline semiconductor film having a sufficient uniformity even when using the laser annealing methods of the Patent References 1 and 2. Particularly, the display apparatus described above includes a TFT array substrate in which a plurality of TFTs are provided for each pixel, but the conventional laser annealing method does not allow suppressing minor variations of TFT characteristics between pixels. This presents a problem of being unable to realize a display apparatus having higher display performance.
In addition, as the conventional layer annealing method, for example, methods as shown in FIGS. 15 to 17 are also used. FIG. 15 is a diagram showing a long-axis profile and a short-axis profile of a layer beam in the conventional layer annealing method.
The light intensity distribution of the laser beam shown in FIG. 15 has: a flat-topped laser energy intensity in the long axis, and a Gaussian laser energy intensity in the short axis.
FIGS. 16A and 16B are diagrams each showing a laser beam scanning method in the conventional laser annealing method.
As shown in FIGS. 16A and 16B, the conventional laser annealing method is a method in which: in the TFT array substrate 200 in which a plurality of pixels 20 are arranged in a matrix, the amorphous semiconductor film formed above the TFT array substrate is repeatedly irradiated with a laser beam having a light intensity distribution as shown in FIG. 16, per unit of a block made up of rows of pixels. For example, as shown in FIG. 16A, beam scanning is sequentially performed on a pair of two rows (two lines) each time, starting from the top, in order of first scanning, second scanning, (k−1)th scanning, and (k)th scanning.
In this processing, the laser beam scanning is repeated as shown in FIG. 17, using a laser beam having the light intensity shown in FIG. 15. In other words, the amorphous semiconductor film is crystallized by irradiating, with the laser beam, the amorphous semiconductor film above the TFT array substrate such that a column direction of the pixels coincides with a long-axis direction of the light intensity distribution of the laser beam. Note that FIG. 17 shows two TFTs for each pixel 20, and also shows source and drain electrodes for each of the TFTs; however, this illustration is intended to clearly indicate the position of the TFTs in the pixel, and therefore when actually performing the laser annealing as described above, neither the source electrode nor the drain electrode is formed yet, nor is the channel layer patterned yet.
Conventionally, the amorphous semiconductor film is thus crystallized, but in the laser annealing method shown in FIG. 17, the laser energy intensity, as shown in FIG. 15, is not uniform in a top portion of the light intensity distribution in the long axis of the laser beam. Because of this, when performing laser irradiation on the amorphous semiconductor film, the laser energy intensity is higher in one end portion of a laser irradiation width than in the other end portion of the laser irradiation width.
This accordingly causes difference between an intensity of the laser energy projected onto pixels in a bottom line of one block an and an intensity of the laser energy projected onto pixels in a top line of a block next to the one block. This varies laser energy intensity, with an extreme difference as shown in FIG. 17, between the pixels in the bottom line of the one block and the pixels in the top line of the next block. In other words, the energy intensity is not continuous when the intensity distribution of the energy projected onto the entire amorphous semiconductor film is viewed in a column direction of the pixels.
As a result, in the crystalline semiconductor film formed by laser irradiation, difference is caused in crystal grain size between the crystalline semiconductor film formed on the gate electrode corresponding to the pixels in the bottom line of the one block and the crystalline semiconductor film formed on the gate electrode corresponding to the pixels in the top line of the next block. This variation in grain size is manifested as variations in TFT characteristics in a boundary between the one block and the next block, and causes a problem of a stripe or line appearing in the display image of each block (block boundary) when viewed as the entire display apparatus.
Note that no matter what is done to flatten a top portion of the light intensity distribution in the long axis shown in FIG. 15, that is, even with attempts to remove the difference in laser energy intensity at both ends of the top portion, such difference will be caused in practice even if only a little.
Thus, the conventional laser annealing method has a problem of not being able to remove the variation in TFT characteristics between pixels.